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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<h1>
<center>
LLVM: Frequently Asked Questions
</center>
</h1>
<hr>
<!--=====================================================================-->
<h2>
<a name="license">Licenses</a>
</h2>
<!--=====================================================================-->
<dl compact>
<dt> <b>Why are the LLVM source code and the front-end distributed
under different licenses?</b>
<dd>
The C/C++ front-ends are based on GCC and must be distributed under
the GPL. Our aim is to distribute LLVM source code under a <em>much
less restrictive</em> license, in particular one that does not
compel users who distribute tools based on modifying the source to
redistribute the modified source code as well.
<p>
<dt><b>Does the University of Illinois Open Source License really qualify
as an "open source" license?</b>
<dd>Yes, the license is
<a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a>
by the Open Source Initiative (OSI).
<p>
<dt> <b>Can I modify LLVM source code and redistribute the modified
source?</b>
<dd>
Yes. The modified source distribution must retain the
copyright notice and follow the three bulletted conditions listed in
the <a href="http://llvm.cs.uiuc.edu/releases/1.0/LICENSE.TXT">LLVM license</a>.
<p>
<dt> <b>Can I modify LLVM source code and redistribute binaries or
other tools based on it, without redistributing the source?</b>
<dd>
Yes, this is why we distribute LLVM under a less restrictive license
than GPL, as explained in the first question above.
<p>
</dl>
<hr>
<!--=====================================================================-->
<h2>
<a name="source">Source Code</a>
</h2>
<!--=====================================================================-->
<dl compact>
<dt> <b>In what language is LLVM written?</b>
<dd>
All of the LLVM tools and libraries are written in C++ with extensive use
of the STL.
<p>
<dt><b>How portable is the LLVM source code?</b>
<dd>
The LLVM source code should be portable to most modern UNIX-like operating
systems. Most of the code is written in standard C++ with operating
system services abstracted to a support library. The tools required to
build and test LLVM have been ported to a plethora of platforms.
<p>
Some porting problems may exist in the following areas:
<ul>
<li>The GCC front end code is not as portable as the LLVM suite, so it
may not compile as well on unsupported platforms.
<p>
<li>The Python test classes are more UNIX-centric than they should be,
so porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will
require some effort.
<p>
<li>The LLVM build system relies heavily on UNIX shell tools, like the
Bourne Shell and sed. Porting to systems without these tools (MacOS 9,
Plan 9) will require more effort.
</ul>
</dl>
<hr>
<!--=====================================================================-->
<h2>
<a name="build">Build Problems</a>
</h2>
<!--=====================================================================-->
<dl compact>
<dt><b>When I run configure, it finds the wrong C compiler.</b>
<dd>
The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and
then <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and
<tt>CXX</tt> for the C and C++ compiler, respectively.
If <tt>configure</tt> finds the wrong compiler, either adjust your
<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
explicitly.
<p>
<dt><b>I compile the code, and I get some error about /localhome</b>.
<dd>
There are several possible causes for this. The first is that you
didn't set a pathname properly when using <tt>configure</tt>, and it
defaulted to a pathname that we use on our research machines.
<p>
Another possibility is that we hardcoded a path in our Makefiles. If
you see this, please email the LLVM bug mailing list with the name of
the offending Makefile and a description of what is wrong with it.
<dt><b>The <tt>configure</tt> script finds the right C compiler, but it
uses the LLVM linker from a previous build. What do I do?</b>
<dd>
The <tt>configure</tt> script uses the <tt>PATH</tt> to find
executables, so if it's grabbing the wrong linker/assembler/etc, there
are two ways to fix it:
<ol>
<li>Adjust your <tt>PATH</tt> environment variable so that the
correct program appears first in the <tt>PATH</tt>. This may work,
but may not be convenient when you want them <i>first</i> in your
path for other work.
<p>
<li>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that
is correct. In a Borne compatible shell, the syntax would be:
<p>
<tt>PATH=<the path without the bad program> ./configure ...</tt>
<p>
This is still somewhat inconvenient, but it allows
<tt>configure</tt> to do its work without having to adjust your
<tt>PATH</tt> permanently.
</ol>
<dt><b>When creating a dynamic library, I get a strange GLIBC error.</b>
<dd>
Under some operating systems (i.e. Linux), libtool does not work correctly
if GCC was compiled with the --disable-shared option. To work around this,
install your own version of GCC that has shared libraries enabled by
default.
<p>
<dt><b>I've updated my source tree from CVS, and now my build is trying to
use a file/directory that doesn't exist.</b>
<dd>
You need to re-run configure in your object directory. When new Makefiles
are added to the source tree, they have to be copied over to the object
tree in order to be used by the build.
<p>
<dt><b>I've modified a Makefile in my source tree, but my build tree keeps
using the old version. What do I do?</b>
<dd>
If the Makefile already exists in your object tree, you can just run the
following command in the top level directory of your object tree:
<p>
<tt>./config.status <relative path to Makefile></tt>
<p>
If the Makefile is new, you will have to modify the configure script to copy
it over.
<p>
<dt><b>I've upgraded to a new version of LLVM, and I get strange build
errors.</b>
<dd>
Sometimes changes to the LLVM source code alters how the build system
works. Changes in libtool, autoconf, or header file dependencies are
especially prone to this sort of problem.
<p>
The best thing to try is to remove the old files and re-build. In most
cases, this takes care of the problem. To do this, just type <tt>make
clean</tt> and then <tt>make</tt> in the directory that fails to build.
<p>
<dt><b>I've built LLVM and am testing it, but the tests freeze.</b>
<dd>
This is most likely occurring because you built a profile or release
(optimized) build of LLVM and have not specified the same information on
the <tt>gmake</tt> command line.
<p>
For example, if you built LLVM with the command:
<p>
<tt>gmake ENABLE_PROFILING=1</tt>
<p>
...then you must run the tests with the following commands:
<p>
<tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt>
<p>
<dt><b>Why do test results differ when I perform different types of
builds?</b>
<dd>
The LLVM test suite is dependent upon several features of the LLVM tools
and libraries.
<p>
First, the debugging assertions in code are not enabled in optimized or
profiling builds. Hence, tests that used to fail may pass.
<p>
Second, some tests may rely upon debugging options or behavior that is
only available in the debug build. These tests will fail in an optimized
or profile build.
</dl>
<hr>
<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
<br>
</body>
</html>
|
